The advance of liquid crystal displays (LCDs) of recent date has been remarkable. LCDs have been widely used for small displays of mobile equipments, PC monitors, liquid crystal television receivers, and so on. As liquid crystal displays advance, demands for quicker response and higher contrast are increasing.
There are many types of liquid crystal display modes. Among them, a mode which primarily assures high contrast is the vertical alignment mode using liquid crystal with negative dielectric anisotropy. Under cross nicols, in a liquid crystal display in the vertical alignment mode, the longitudinal axis direction of liquid crystal molecules sandwiched between a pair of substrates is orthogonal to the substrates when no electric field is applied. The liquid crystal display is therefore in dark state. On the other hand, when an electric field orthogonal to the substrates is applied, the longitudinal axis direction of the liquid crystal molecules falls from the direction orthogonal to the substrates to the direction in parallel to the substrates. The liquid crystal display therefore becomes in bright state.
An alignment film used in the vertical alignment mode is a vertical alignment film which arranges the longitudinal axis direction of the liquid crystal molecules to be substantially orthogonal to the substrates when no electric field is applied. The vertical alignment film, however, typically cannot control the direction of tilt of the liquid crystal molecules when an electric field is applied. If no countermeasure is taken, the liquid crystal molecules fall in random directions when an electric field is applied, and the brightness is impaired due to the occurrence of disinclination. To solve this problem, various measures have conventionally been taken. Examples of the measures are as follows: a structure is provided between a pair of substrates so that the direction to which liquid crystal molecules falls in response to voltage application is regulated; a chiral material is added so that, in response to voltage application, liquid crystal molecules are arranged to fall while being twisted in a predetermined direction; and the vertical alignment film is rubbed so that the direction in which the liquid crystal molecules fall in response to electric field application is controlled.
As one type of the vertical alignment mode, there is a twisted vertical alignment mode in which, when a voltage is applied, liquid crystal molecules are twisted (i.e. the alignment vector of the liquid crystal molecules in proximity to one substrate is different from the alignment vector of the liquid crystal molecules in proximity to the other substrate.) The alignment directions of the liquid crystal molecules, for example, form an angle of 90° between the respective substrates. This twisted vertical alignment mode makes it possible to achieve a considerably high contrast.
Non-Patent Document 1 discloses a so-called vertically aligned twisted nematic liquid crystal display element. This liquid crystal display element operates in the vertical alignment mode in which liquid crystal with negative dielectric anisotropy is aligned to be substantially orthogonal to the panel surface of a liquid crystal display element, and is arranged such that the liquid crystal molecules are twisted in response to voltage application to the liquid crystal layer and only one of the alignment films has been rubbed.
[Non-Patent Document 1]
T. Takahashi et al. “Electrooptical Properties of Vertically Aligned Twisted Nematic Cells with Negative Dielectric Anisotropy”, Jpn. J. Appl. Phys. Vol. 36 (1997) Part 1, No. 6A, June 1997, p. 3531-3536